Generally, a rolling bearing such as a cylindrical roller bearing or an angular contact ball bearing is used as a main shaft bearing of a machine tool. A plastic retainer formed of a synthetic resin is used as a retainer of these rolling bearings. The plastic retainer is lightweight, and accordingly a centrifugal force during rotation is small, which is advantageous for high-speed rotation.
As a lubrication method of such a rolling bearing, grease lubrication, oil air lubrication, jet lubrication, and the like are selected as appropriate, and generally, grease enclosing lubrication by initial enclosing is widely used due to low cost and easy maintenance (see, for example, Patent Document 1).
In a rolling bearing including a retainer of an outer ring guide type, an axial groove may be provided on a retainer outer diameter surface serving as a guide surface, so as to discharge excess grease in vicinity of a rolling contact portion. When a groove is provided on the retainer outer diameter surface in an axial direction, grease is scraped out in vicinity of a step portion of the groove. On the other hand, when no axial groove is provided in the retainer outer diameter surface, grease is sheared and discharged from the guide surface by rotation of the retainer when entering a place between the retainer and an outer ring inner diameter surface, since a guide gap formed between the outer ring inner diameter surface and the retainer outer diameter surface is constant.